Priming is a form of implicit memory, and refers to the fact that the mere processing of an item can facilitate subsequent processing of that same item. Priming effects can occur in the absence of conscious recollection of the prior study episode, and it has, therefore, been suggested that priming and explicit memory are mediated by different memory systems. Evidence from memory disordered patients is critical for this hypothesis, since if it is possible to link the breakdown of explicit memory and priming to damage to different brain regions this would strengthen the independent memory systems hypothesis. Many studies have found normal priming effects in amnesic patients and some investigators have reported impaired priming in patients with Alzheimer's disease. However, many difficulties are associated with evaluating priming effects in memory impaired subjects. The priming measures may be less sensitive than explicit memory measures, and the lack of group differences may simply reflect low measurement sensitivity. The degree of priming obtained in a given task is related to processing efficiency (i.e. baseline performance), with less efficient processing resulting in greater priming. Therefore, if patients have even mild processing deficits this may mask priming impairments. The proposed project consists of a series of experiments designed to manipulate baseline performance level and sensitivity of the priming measures. Normal control subjects and three groups of patients will be investigated; patients with probable Alzheimer's disease, patients with Huntington's disease, and amnesic patients. These patient groups differ in the severity of their explicit memory defects, and in the extent to which lexical/perceptual processing efficiency is compromised. Younger normal subjects will also be included in the study. Therefore, it will be possible to determine if the decline in memory function resulting from amnesia or dementia is qualitatively similar to the decline in function associated with the normal aging process. In addition to the behavioral measures, measures of volume loss in specific brain structures will be obtained from MRI. These brain measures will be related to the behavioral indices, using a multiple regression approach, to determine the specific role of each brain region in perceptual/lexical processing, explicit memory, and strength of priming. Specific hypotheses about the relationship between the behavioral and structural brain measures derived from our previous work will be tested.